About 2 years later, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) remains a major global threat and concern to humans since its first appearance.
(1) The acute disease caused by this virus, the coronavirus disease 2019 (COVID-19), is also still ongoing with confirmed infections and deaths to more than 260 and 5.18 million, respectively, people worldwide due to this universal pandemic.
(2) The increasing evolution of very resistant and virulent novel variants/strains of the COVID-19 virus, especially in the current year 2021, puts a very heavy load on the concerned scientific communities in all of the countries to become much more active in searching for and finding successful medicines and vaccines effective in inhibiting and fighting this irritating virus, along with finding drugs having the abilities to counteract all, or most of, the very severe effects of the COVID-19 on human bodies; thus, searching for efficient comprehensive or dual-action anti-SARS-CoV-2/anti-COVID-19 therapeutic agents will be extremely advantageous as an excellent solution for this untreatable infection and its health sequelae.
(3) Tens of new and repurposed promising compounds are nowadays under broad international/multinational investigations (including
in vitro,
in vivo, and clinical trials) to be biologically evaluated as effective candidate anti-COVID-19 drugs,
e.g., remdesivir (GS-5734; FDA-approved administration for emergent use), GS-441524, GS-443902, favipiravir, cyanorona-20, hydroxychloroquine, chloroquine, CoViTris2020, taroxaz-104, ChloViD2020, teriflunomide, leflunomide, ivermectin, arbidol, and colchicine, but none of them has proved its successful eventual broad-spectrum effectiveness until now (
i.e., the final results of almost all of these worldwide investigations are not disclosed to date).
(4−20) Nucleoside and nucleotide analogism is among the top therapeutic options in drug designers’ and medicinal chemists’ minds to combat and stop coronavirus replication inside the human body.
(4−9,21) In this COVID-19 therapeutic option or strategy, the natural or synthetic nucleoside/nucleotide analogue uses its resemblance to the normal nucleosides and nucleotides to mislead and trick the SARS-CoV-2 RNA-dependent RNA polymerase (RdRp) through incorporation and embedding in the viral genetic strands instead (
i.e., in place) of the normal naturally occurring nucleosides and nucleotides, leading to repeated excessive ambiguous coding and premature termination of mRNA synthesis with the generation of vague RNA strands at the end; these pseudostrands, in turn, form abnormal noninfectious and inactive viral particles, and hence no further replication of the virus occurs (
Figure 1).
(21) Some of the aforementioned investigational anti-COVID-19 agents, such as remdesivir and cyanorona-20 and their active metabolites GS-441524 and favipiravir, respectively, depend on this tactical mechanism in their inhibitory bioactivities on the SARS-CoV-2 (
Figure 2).
(4−9) Unfortunately, all the four compounds are synthetic and did not give satisfactory results in the
in vitro/
in vivo anti-SARS-CoV-2 activity assays (except cyanorona-20, which has a significant anti-SARS-CoV-2 EC50 value of 0.45 μM); therefore, searching for more potent natural anti-COVID-19 drugs is very demanding.
(4−9) Figure 1
Figure 1. Representation of the nucleoside/nucleotide analogism approach used for the potent blockade of the SARS-CoV-2 replication in COVID-19 therapy.
Figure 2
Figure 2. Chemical structures of remdesivir, GS-441524, cyanorona-20, and favipiravir.
Cordycepin is a known unique natural adenosine analogue found and extracted from various species in the fungal kingdom, mainly those from the two genera
Cordyceps (
e.g., the species
Cordyceps militaris) and
Ophiocordyceps (
e.g., the species
Ophiocordyceps sinensis).
(22−25) Chemically, the compound cordycepin is 3′-deoxyadenosine (3-dA), which is IUPACally named as (2
R,3
R,5
S)-2-(6-amino-9
H-purin-9-yl)-5-(hydroxymethyl)oxolan-3-ol or 9-(3-deoxy-β-d-ribofuranosyl)adenine (
Figure 3).
(26) Recently, the natural cordycepin content was biologically increased to more than twofold in the entomopathogenic medicinal fungus
C. militaris using newly designed techniques of optimized cultivation and fermentation, which significantly enhanced the biosynthesis of this precious bioactive metabolite.
(27) Cordycepin could also be easily synthesized from many readily available simple starting materials, such as d-glucose, d-xylose, and adenosine, using different chemical synthetic routes.
(28,29) Cordycepin molecule is specifically a nucleosidic adenosine molecule that lacks only one hydroxyl group at the 3′ position of the five-membered ring of its ribose moiety (
Figure 3).
(26,30) This extreme structural similarity with the cellular nucleoside adenosine renders cordycepin biologically acting on adenosine receptors and very similar to this pivotal nucleoside in all its biological activities in living organisms. Additionally, this structural analogism also provides camouflage abilities to the cordycepin molecule to be able to antagonize/block and impair/disrupt many of the biological actions of the adenosine molecule itself. Cordycepin has a very broad spectrum of diverse important pharmacological activities since it effectively acts as, for example, antiviral (also in viral myocarditis), antifungal, antibacterial, antitubercular, antimalarial, antiprotozoal, antimicrobial, insecticidal, larvicidal, antiinflammatory, antioxidant (and in several oxidative stress statuses of many different diseases), immunomodulatory/immunoregulatory, antileukemic (since it has exhibited promising
in vitro cytotoxicities against various leukemic cell lines), antitumor/anticancer (antineoplastic), antiproliferative, antifibrotic, antimetastatic (in cancer therapy), apoptosis inducer (in cancer cells), antihyperglycemic/antidiabetic, antihyperlipidemic (as in cardiovascular diseases and vascular disorders), antihypercholesterolemic, antiarrhythmic, antitachycardic, coronary vasodilator, antihypertensive, angiogenic, antiatherosclerotic, antiplatelet aggregation, antithrombotic/thrombolytic/fibrinolytic, anti-ischemic (as in myocardial infarction/ischemia and cerebral ischemia injuries), reperfusion therapy, antistroke, hepatoprotective (as in hepatitis and liver cirrhosis), renal functions improver/nephroprotective (renoprotective and in renal fibrosis and chronic kidney disease (CKD)), chondrogenesis promoter, antiarthritic (as in osteoarthritis), antiosteoporotic (in bone loss), antihyperuricemic, antirheumatic (as in multiple sclerosis (MS) and rheumatoid arthritis (RA)), intervertebral disc regenerator, bronchial asthma reliever, acute lung inflammation/injury healer (as in acute respiratory distress syndrome (ARDS), cystic fibrosis (CF), and chronic obstructive pulmonary disease (COPD)), pneumoprotective, cough/common cold suppressant, antihypoxic, antidepressant, nootropic, energizer, tonic, stimulant, mood modulator, natural endurance booster, pain killer/analgesic, antifatigue, anticognitive dysfunction, erythropoiesis stimulator, zincophoric (zinc ion carrier), neuronal regenerator, antiparkinsonian, antisleep disorders, antiaging, sexual enhancer, aphrodisiac (prosexual), spermatogenic, anti-infertility, benign prostatic hyperplasia (BPH) inhibitor, steroidogenetic (as in testosterone/estrogens biosyntheses), intestinal irritation suppressor (as in acute colitis conditions), some toxins antidote, and cosmeceutical (in skin photoaging and other cosmeceutical skin/hair problems, also in inflammatory skin conditions/diseases like atopic dermatitis).
(31−36) All of the aforementioned interesting biological activities make cordycepin one of the most important promising phytotherapeutic agents. Cordycepin is already being clinically investigated as a potential effective antileukemic/anticancer chemotherapeutic agent of the antimetabolites class since 1997 until now in several clinical settings worldwide to pass the clinical phases 1 and 2 (
e.g., clinical trials NCT00003005 and NCT00709215).
(37,38) Figure 3
Figure 3. Chemical structures of cordycepin and adenosine.
Most of the previously mentioned diverse biological activities of cordycepin are extremely needed in the comprehensive treatment of COVID-19; this makes cordycepin a very promising potential comprehensive anti-COVID-19 drug rather than being only a candidate SARS-CoV-2 inhibitor (the complementary part that was proven in the current study).
(17) The antiviral activities of cordycepin are very potent and broad since they cover several species of human viruses (including almost all virulent RNA viruses, especially those belonging to the genus
Flavivirus),
e.g., adenoviruses (AVs), dengue virus (DENV), Epstein-Barr virus (EBV), hepatitis B virus (HBV), hepatitis C virus (HCV), Herpes simplex virus 1 and 2 (HSV-1 and HSV-2), human immunodeficiency virus (HIV), human poliovirus (HPV), human rhinovirus (HRV), influenza viruses (IVs), Kyasanur Forest disease virus (KFDV), murine leukemia viruses (MLVs or MuLVs), murine sarcoma viruses (MSVs or MuSVs), Newcastle disease virus (NDV), Omsk hemorrhagic fever virus (OHFV), Powassan virus (POWV), rotaviruses (RV), vaccinia virus (VACV or VV), West Nile virus (WNV), yellow fever virus (YFV), and Zika virus (ZIKV).
(33,34,39−43) These strong inhibitory activities extend to some plant viruses, such as cowpea chlorotic mottle virus (CCMV) and tobacco mosaic virus (TMV).
(44,45) The strong antiviral actions of cordycepin are mediated through specific molecular mechanisms that mainly originate due to the extreme structural analogism with the nucleoside adenosine, as this analogism renders the human biological system unable to recognize this molecule,
e.g., many enzymes fail to discriminate between it and the endogenous adenosine.
(33,36,46) Through this camouflage, administered cordycepin could easily be involved in successfully inhibiting/blocking certain several biochemical pathways and reactions in the human body and viral particles; for example, it could cause potent poly(A) polymerase inhibition (
i.e., acts as a polyadenylation inhibitor), severe shortening of poly(A) tails, continuous destabilization of mRNAs, strong purine biosynthesis inhibition, and also premature termination of protein synthesis.
(33,36,46) A recently published study sheds light on the possible potent inhibitory effects of cordycepin against SARS-CoV-2 RdRp using an
in silico computational molecular docking approach.
(47) The evaluation results of this validated simulation protocol clearly revealed the very strong intermolecular interactions with the most vital amino acid residues of the principal active site of the SARS-CoV-2 RdRp, proposing that cordycepin molecule actively interacts with the giant molecule of RdRp (
Figure 4).
(47) These strong inhibitory interactions were reflected in the resultant relatively low binding energy, which reaches about −8.2 kcal/mol at the best pose of docking, with the formation of a very stable cordycepin–RdRp complex.
(47) Interestingly, it was found that the cordycepin molecule strikes and binds to the two pivotal catalytic amino acid residues, Asp760 and Asp761, of the active site of the SARS-CoV-2 RdRp
via both strong hydrogen bonds and hydrophobic interactions.
(47) Additionally, the cordycepin molecule was found to bind also to some residues that are very close to the catalytic ones in the active site,
e.g., Trp617, Asp618, Trp800, and Glu811 residues.
(47) These additional intermolecular interactions are mediated
via diverse bonds/interactions, such as nonbonding interactions, hydrogen bonds, and hydrophobic interactions.
(47) Figure 4
Figure 4. Molecular docking output image showing the best expected binding mode of cordycepin to SARS-CoV-2 RdRp according to the novel hypothesis of Bibi et al. (47)
Herein, we suggest that cordycepin could effectively inhibit the coronaviral replication (
i.e., act as a SARS-CoV-2 replication inhibitor) by significantly reducing the number of replicated coronaviral copies, which is expected to result from locking mainly the genomic RNA synthesis occurred
via the SARS-CoV-2 RdRp, using the nucleoside analogism strategy (as previously mentioned). In this effective mechanism of anti-RNA action, the nucleoside-like cordycepin molecule is first readily phosphorylated to its mono-, di-, and triphosphate forms (
i.e., its nucleotide analogues) intracellularly, then the superactive nucleotide analogue cordycepin triphosphate (Cor-TP) can be easily incorporated into RNA in place of the endogenous chemicosimilar bionucleotide adenosine triphosphate (ATP); this inhibits and terminates transcription elongation and synthesis of viral RNA strands in all stages (
i.e., acts as an RNA elongation inhibitor due to the hydroxyl moiety absence at the 3′ position of the molecule, this one-hydroxyl group deficiency significantly antagonizes the SARS-CoV-2 RdRp activity as previously mentioned), giving incomplete disrupted premature RNAs in the developing mRNA strands and viral genomes, and finally, this ambiguous coding leads to strong inhibition of SARS-CoV-2 copying/replication and generation of inactive, noninfectious, mutated, useless, nonpathogenic, and nonviral/non-SARS-CoV-2 particles instead of the active, parent, correct, original, infectious, pathogenic, and virulent SARS-CoV-2 particles (
Figure 5). This currently presented logic hypothesis, which proposes the potential strong anti-SARS-CoV-2/anti-COVID-19 properties of the cordycepin molecule, was actually tested and evaluated in this current work through a validated anti-SARS-CoV-2 bioassay (along with a full toxicological evaluation). Based on all of the previous promising literature data together with the very interesting
in silico and
in vitro (
i.e., biological) findings of the current research, cordycepin can be repurposed to
in vivo evaluate its protective and inhibitory activities specifically against SARS-CoV-2 particles’ invasion (as an anti-RNA virus agent) and to clinically evaluate its protective and inhibitory activities comprehensively against COVID-19 status as a whole (as an anti-COVD-19 condition agent) in COVID-19 patients.
Figure 5
Figure 5. Illustration of the newly proposed mechanism of anti-SARS-CoV-2 action of cordycepin.
3. Conclusions and Future Recommendations
COVID-19 treatment is still a critical challenge to all respective scientists. Cordycepin is a promising potential comprehensive anti-COVID-19 agent that should be put under the microscope in the coming days. Some new studies reported the
in silico inhibitory activities of cordycepin on SARS-CoV-2 spike (S) protein and main protease (Mpro) enzyme,
(46,56,57) and, recently, another computational study showed the
in silico inhibitory activities of cordycepin on the SARS-CoV-2 RdRp enzyme.
(47) In light of the findings of the previous
in silico studies and the current
in vitro study, it is proven that the cordycepin molecule has interesting superiority and advantage over most of the other investigational anti-SARS-CoV-2 molecules, especially those acting only on the S protein and/or Mpro enzyme, as, first, it acts
via the expected synergistic triple anti-SARS-CoV-2 mode of action (
i.e., it acts on the three different target proteins, thus inhibiting all the three major stages of the SARS-CoV-2 infection in humans, which are the viral entry, replication/multiplication, and pathogenic phases) and, second, it acts as nonspecific/nonselective anti-SARS-CoV-2,
i.e., it is capable of acting on all SARS-CoV-2 strains and variants since its anti-SARS-CoV-2 bioactivity does not depend only on the predicted action on the changeable and mutated S protein (cordycepin has general broad-spectrum anti-SARS-CoV-2 activities effective on all SARS-CoV-2 strains and variants with different mutations).
The expected comprehensive nature of cordycepin in COVID-19 treatment mainly comes from two practically proven pathways. First, it can inhibit the replication and survival of the COVID-19-causing virus itself with potent broad-spectrum activities (including activities against the most recent strains of SARS-CoV-2), which reach an EC50 of about 2 μM. This anti-SARS-CoV-2 action of cordycepin is mainly due to its chemical similarity/analogism with the human adenosine. During viral replication, the SARS-CoV-2 RdRp incorporates Cor-TP into the newly synthesized RNA strands instead of using real ATP. Consequently, when the SARS-CoV-2 RdRp attempts to copy the destabilized RNA containing Cor-TP, it either falsely interprets it or fails to interpret it. This interrupted and hindered interpretation and the consequent errors in the viral genetic code cause a very significant number of mutations in all downstream coronaviral-2 copies that much exceeds the threshold the virus can survive, an effect known in virology as lethal mutagenesis or viral error catastrophe. Second, it can mitigate and relieve COVID-19 characteristic severe symptoms and sequelae that are mainly related to the SARS-CoV-2 invasion of the respiratory and cardiovascular systems of the patients since this attack results primarily in acute biological disruptions of immunologic/inflammatory origin (see the numerous diverse and broad pharmacological activities of cordycepin in
Section 1).
Promisingly, the net distinction, in almost all anti-COVID-19 parameters/properties/activities, of cordycepin over the potent antiviral agents remdesivir and GS-441524 supports cordycepin candidacy to be a superior potential comprehensive therapeutic agent against COVID-19. It is also worth mentioning that the cordycepin molecule has more than 10 sites that are considered as very active positions for chemical reactions; thus, hundreds of possible derivatives with required enhanced pharmacokinetic and/or pharmacodynamic properties can be designed and synthesized based on the biocompatible scaffold of this golden multitask molecule, which belongs to the nucleoside analogues’ class of antiviral/antineoplastic therapeutic agents. Last but not the least, it is highly suggested to the scientific community to start the cordycepin repurposing journey against COVID-19 by conducting extensive worldwide preclinical and clinical studies to seriously evaluate the efficacy and safety of cordycepin (as a monotherapy or, optionally, as either a double-combination therapy with DPM or a triple-combination therapy with both DPM and PTN) to be used for the comprehensive treatment and prevention of all types of COVID-19 infections.
